This study was designed as a multicenter, double-blind, placebo-controlled, parallel-group, prospectively randomized study comparing, after a 4-week placebo run-in phase, moxonidine 0.4 mg once daily (o.d.), hydrochlorothiazide 25 mg o.d., and the combination of the two with placebo. A total of 160 patients were analyzed in an intent-to-treat analysis. Moxonidine 0.4 mg o.d. was effective in significantly lowering blood pressure in this group of mild-to-moderate hypertensive patients in comparison with placebo. The efficacy and the side-effect profile of moxonidine were comparable to those of the first-line antihypertensive agent hydrochlorothiazide. The combination of moxonidine and hydrochlorothiazide in the same dosage as a monotherapy improves efficacy significantly without additive effects on the safety profile. Response rate after monotherapies was calculated with 70.3 and 70.0%, respectively, after combination treatment in 87.8% of all patients in the treatment group. The trial gives support to a recommended dosage regimen of moxonidine 0.4 mg o.d. This profile of moxonidine is highly comparable to a standard first-line antihypertensive drug such as hydrochlorothiazide, without sacrificing tolerance and safety for increased efficacy, in combination with hydrochlorothiazide.